1. Field of the Invention
The present invention relates to processing technology for image information which includes a first image component which was responsive to an image-forming light and a second image component which was responsive to a non-image forming light, and more specifically to image processing technology to make a second image component inconspicuous.
2. Description of the Related Art
Optical elements which have a diffractive surface (diffractive optical elements) are elements which actively utilize the diffraction effect of light, and have an action which separates light that is incident on the diffractive optical element into a variety of diffraction orders or an action which condenses only light of a predetermined diffraction order, and the like.
For example, characteristics of a diffractive optical element designed to have a condensing action include a small thickness, a capability of generating an aspherical wave, and a dispersion of a negative value in the lens design. Therefore, although compact, it is useful for correcting an aberration in an optical system, and has an advantage of being able to obtain a so-called “achromatic effect.”
Light which is incident on a diffractive optical element is separated into diffracted light of a plurality of orders. Only one of these components is used in image formation, that is, there is only one image-forming order, and the components of the other orders exist as unwanted diffraction-order light (hereunder, referred to as “unwanted diffracted light”) which have a focal point in a position which is different to that of the image-forming order component and are present as flare on an image surface.
Therefore, although there are cases where a contrivance is devised whereby a diffractive surface is rendered serrate to condense light only in an image-forming order, it is difficult to suppress unwanted diffracted light across an entire visible light region. Accordingly, the practical application thereof as an element which comprises an image-taking optical system such as a camera lens has been difficult.
To overcome this problem, as shown in FIG. 1, a technique has been proposed which reduces the diffraction efficiency of unwanted orders significantly more than the conventional monolayer-type diffractive optical elements by superimposing diffractive optical elements 201 and 202 which have respective diffractive surfaces 201a and 202a. By use of this layer-type diffractive optical element (DO lens) 200, it is possible to produce a compact camera lens which maintains high image formation performance while favorably correcting chromatic aberration.
However, even when using a DO lens, when taking an image of an object of an extremely intense brightness (luminous energy), the intensity of unwanted diffracted light is raised by the intense brightness and results in a large intensity. For example, in a given DO lens, as shown in FIG. 2, the image-forming properties of diffraction order light rays (hereunder, referred to as “image-forming diffracted light”) used for image formation with respect to the highest brightness value of the image-forming properties of unwanted diffracted light is taken as 10000:1. In the case of using this lens when installed in a camera that is capable of acquiring an 8-bit digital image and taking an image of a point source of light in a dark room, when exposure is set such that an image produced by image-forming diffracted light is taken within the range of 8 bits, the highest brightness of flare produced by unwanted diffracted light is 256/10000=0.0256, and flare hardly appears in the image that is taken.
However, when the exposure is increased 10000-fold, the brightness of pixels which correspond to an image produced by image-forming diffracted light is completely saturated, and the highest brightness of flares produced by unwanted diffracted light becomes 256, such that, as shown in FIG. 3, the flares form an image that is clearly distinguishable in regions in which the flares do not overlap with the image produced by the image-forming diffracted light.
Thus, depending on the image-taking environment, the occurrence of flare images due to unwanted diffracted light is an unavoidable phenomenon.
Meanwhile, in a digital image-taking apparatus such as a digital camera, image information of an object image acquired by a photoelectric conversion element such as a CCD or CMOS is subjected to image processing which converts the image information into a visually acceptable image.
In Japanese Patent Laid-Open No. H09-238357 and Japanese Patent Laid-Open No. H11-122539 a technique is proposed which corrects flare produced by unwanted diffracted light in a digital image-taking apparatus which uses an optical system which has a diffractive surface. The technique involves calculating as flare images, flare components produced by unwanted diffracted light generated by a diffractive surface in a taken image, by multiplying image-forming properties of unwanted diffraction orders in that image by a diffraction efficiency weighting and subtracting the result from the taken image, to bring the taken image itself close to the component for the object image.
However, in a practical DO lens, the image-forming order is sufficiently larger than the diffraction efficiency of unwanted orders, and thus unwanted diffracted light does not become visually noticeable in a taken image at a brightness where the brightness of an image produced by the image-forming order is not saturated. Therefore, the technology proposed in the above publications is meaningless when using a practical DO lens for this type of image-taking.
Moreover, when flare resulting from unwanted diffracted light appears in a taken image even when using a practical DO lens, it indicates that the brightness of the image produced by the image-forming light is definitely saturated. Thus, even if the technique proposed in the aforementioned patent literature is used, since the object image component itself cannot be accurately obtained because of brightness saturation, a flare component that is to be corrected cannot be estimated.
Further, in Japanese Patent Laid-Open No. 11-122539 a technique is proposed for cases in which the above kind of brightness saturation occurs which calculates an object image component on the basis of a taken image in which brightness is not saturated, by conducting almost simultaneous image-taking of an object a plurality of times at different exposures. However, in practice it is extremely difficult to conduct image-taking almost simultaneously a plurality of times in a state where there is no means to know what level of exposure to use to suppress brightness saturation, and in particular, it is not a useful means for a moving object. There is also a possibility that the exposure set in the image-taking apparatus will not be capable of suppressing brightness saturation.
Because of the above problems, for images which include highlights to the extent that unwanted diffracted light constitutes a problem, in reality it is not possible to take images in which the problem of brightness saturation is overcome with respect to the brightness distribution of the image-forming diffracted light unless the image is taken in a special environment, and this is not useful as a technique which eliminates unwanted diffracted light.